Drilling operations typically involve mounting a drill bit on the lower end of a drill pipe or drill stem and rotating the drill bit against the bottom of the hole to penetrate the formation, creating a borehole. A drilling fluid, typically referred to as drilling mud may be circulated down through the drill pipe, out the drill bit, and back up to the surface through the annulus between the drill pipe and the annular wall. The drilling fluid has a number of purposes including cooling and lubricating the bit, carrying the cuttings from the hole to the surface, and exerting a hydrostatic pressure against the borehole wall to prevent the flow of fluids from the surrounding formation into the borehole.
A drilling fluid can place undesirable mechanical stress on the rock around the wellbore and may even damage the reservoir. With increasing depth a hydrostatic pressure acts outwards on the borehole, which may cause mechanical damage to the formation and reduce the ability of the well to produce oil or gas. Drilling fluids may also fracture the formation, requiring a drilling shutdown in order to seal the fracture.
It is therefore necessary to stabilize the formation and secure the wellbore from time to time.
After a section of the wellbore has been drilled, drilling operations are stayed or ceased to seal the wellbore using a string of pipe such as casing or a liner in the wellbore. The stops are normally referred to as casing points. At a casing point, a sealing composition such as hydraulic cement slurry is pumped into the annular space between the walls of the wellbore and the exterior of the string of pipe disposed therein. The cement slurry is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the wellbore and bonds the pipe to the wellbore whereby the undesirable migration of fluids between zones or formations penetrated by the wellbore is prevented.
This well-established technique has several disadvantages including a reduction in the well diameter after each casing point and the high cost of the casing itself.
The greatest weakness in conventional cement jobs is the poor bonding to the formation because of the presence of the drilling fluid filter cake on the wellbore wall. This can result in the flow or migration of formation fluids along weak pathways or microannuli in the annular gap between the casing and the formation.
U.S. Pat. No. 4,760,882 discloses a method for drilling and primary cementing using a mud containing a polymeric material that converts to cement on contact with an initiator and optionally radiation. U.S. Pat. Nos. 4,768,593 and 4,547,298 are similar but require radiation for curing. U.S. Pat. No. 4,760,882 gives useful background on mud-to-cement and also a good list of reactive low mol wt polymers including polybutadienes (PBD) and derivatives and a monomeric crosslinking agent containing at least one double bond as being required to cross-link the PBD and other polymers.
U.S. Pat. No. 5,293,938 describes a method of forming in a wellbore a dual-state composite of a hardened hydraulic cement and a solid rubber consisting of hydraulic cement and a vulcanizable rubber latex. The method pumps the mixture in place, allows the cement to set, then causes the latex to vulcanise.
U.S. Pat. No. 5,945,387 Halliburton relates to a settable plug that combines a water-soluble monomer in water with rubber latex as used before. The monomer is polymerised and the latex is vulcanised to a highly pliable and durable mass. No LMW liquid reactive water-insoluble polymers are used and no cement is used. U.S. Pat. Nos. 6,098,711; 6,234,251 and 6,330,917 describe related methods and compositions.
U.S. Pat. Nos. 4,115,336 and 4,204,998 cover a very stable (invert) emulsion of water in unsaturated polyester resin that is then mixed with cement. The resin is polymerised or crosslinked using an unsaturated monomer like styrene and initiated by for example peroxides.
U.S. Pat. No. 3,437,619 is an early description of a cement/polymer composite material formed by polymerising/cross-linking a polyester resin with an unsaturated monomer like styrene whilst simultaneously hydrating Portland cement dispersed in the polyester resin/monomer mixture. The cement is first mixed into the polyester and unsaturated monomer organic liquid phase. The initiator, a powdered water-soluble per-salt like sodium persulphate, is also dispersed into the organic phase.
Because of its insolubility in the organic liquid the initiator does not set off the polymerisation and cross-linking. The mixture remains a workable paste for a period. However when water is added, the initiator is activated by its dissolution and perhaps the high pH, causing the organics to start polymerising concurrently with the cement hydrating.
Thus there is a need for processes and materials that further improve the zonal isolation in a wellbore.